Based on that description, reecardo is absolutely correct and you should have no trouble using the Get HTTP Request Value component. The first field in the component, "Request Variable To Get", should simply contain the name of the variable in the URL. If your URL is going to be "http://server/Workflow/MyForm.aspx?username=george.michael", then you would enter "username" into that field. The second field is simply what you want to refer to that variable as within your workflow. Name it whatever you want and know that in the above case, it would contain the value "michael.george".
It should be noted that if you were using a anything other than a web forms project, this is probably the best way to do things. If you are doing a web forms type project, you could also get away with using the "Input Data" fields to enter in your parameters.